var isSymmetric = function(root) {
    if(root==null) return root
    return pd(root.left,root.right)
};
function pd(left,right){
    if(left==null&&right==null)return true
    if(left==null||right==null)return false
    if(left.val!==right.val)return false
    return pd(left.left.right.right)&&pd(left.right,right,left)
}